In the coating for car bodies, paints using organic solvents are predominant and, with a view point of environmental protection and public pollution prevention, it has been demanded to decrease volatile organic solvents formed in a great amount in a coating step, and coating by aqueous paints has attracted attention as a countermeasure therefor.
For using an aqueous paint with minimum wasteful loss, while it is preferred to conduct coating by an electrostatic coating apparatus of high transfer efficiency, since the aqueous paint has low electric resistance and a rotary atomizing head of the electrostatic coating machine tends to be conducted electrically to the ground side by way of a paint flowing through a paint supply system, it is necessary to apply an insulation countermeasure over the paint supply system thereby preventing leakage of a high voltage at −60 to kV applied to the rotary atomizing head.
Therefore, it has been attempted so far not to leak high voltage even when it is applied to the coating machine by filling a paint from a paint supply system to a coating material filling tank formed in a coating machine or mounted detachably to the coating machine, and then extruding a paint from the filling tank in a state of physically disconnecting the paint supply system and the coating machine and conducting coating.    Patent Document 1: JP-A No. 2000-317354
Usually, in the electrostatic coating machine of this type, a bottom plate as a piston is slidably disposed along the inner peripheral surface of a coating material filling tank as a cylinder and the bottom plate is driven in forcively by other actuator or a liquid pressure to extrude the paint.
However, in this case, since the bottom plate and the inner peripheral surface have to be sealed reliably, friction is increased by so much to require a large driving force and the seal is abraded frictionally on every reciprocation of the bottom plate, a hydraulic fluid flows in a case of a liquid pressure driving to possibly give undesired effects on the paint quality.
Further, in a usual seal, since a plurality of O-rings are arranged in parallel to the outer peripheral surface of the bottom plate as a piston, paints intrude between an O-ring and another O-ring to cause a trouble that the bottom plate has to be detached and decomposed for cleaning upon cleaning after the completion of a day's work.
In view of the above, the present applicant has trially manufactured a coating material filling tank having a soft and flexible coating material bag to be filled with a coating material and conducting a coating experiment by detachably mounting the same to a coating machine having a coating material atomizing mechanism such as a rotary atomizing head.
In the coating material filling tank, a paint (coating material) is previously filled in the coating material bag and, when a hydraulic fluid is supplied at a metered amount to the outside thereof upon conducting coating, the coating material bag is squashed by the liquid pressure and the paint is extruded at a metered amount and supplied to the atomizing mechanism.
Accordingly, since there is no effect such as friction due to the O-rings, the pressure of the hydraulic fluid acts as it is on the coating material and the coating material can be supplied by a relatively small driving force to the atomizing mechanism.
Further, since the bottom plate as the piston is not used, there is no gap for intrusion of the coating material between the bottom plate and the coating material extruding chamber, cleaning can be conducted easily and coating failure was no more caused by insufficient cleaning or seal leakage of the bottom plate.
However, it has been found that bursting of the coating material bag or lack of paint during coating sometimes occurs during use of such coating material filling tank while repeating filling/extrusion of the paint.
Further, the filling amount of the paint to the paint bag is initially set to the amount of use plus for preventing lack of paint during coating. Theoretically, the filling amount of the paint does not exceed the amount of use plus α, or is in short of the determined amount to be used upon starting the coating.
Actually, there may be an error, however, between the actual amount of use and the predetermined amount of use and, in a case where the actual amount of use is less than the predetermined amount of use, a surplus paint corresponding to the error is accumulated and when the bag is used in excess of the capacity thereof, the paint bag is burst.
Particularly, since the coating material bag tends to be worn by repetitive filling/discharging of the paint over and over, in a case where the coating material bag is formed of an extremely soft and flexible material in order to provide durability against filling/discharging, this results in a problem to lower the pressure resistance to the excess filling of the paint.
Further, in a case where the actual amount of use is more than the predetermined amount of use, a paint corresponding to the error is consumed and lack of paint sometimes occurs at the time that the paint which previously filled with a margin was exhausted.